1. Field of the Invention
The present invention relates to an IPS (in-plane switching) mode LCD (liquid crystal display), and the control of liquid crystal molecules therein.
2. General Background
Liquid crystal displays (LCDs) are typically used for information display in various devices such as computers and vehicle and airplane instrumentation. One type of LCD called the twisted nematic liquid crystal display (TN-LCD) often has the drawback of a narrow range of viewing angles. Thus, an improved design called the in plane switching liquid crystal display (IPS-LCD) has been developed in order to provide a broad range of viewing angles.
A conventional IPS mode LCD has an upper substrate, a lower substrate, and a liquid crystal layer interposed therebetween. The liquid crystal layer has a plurality of liquid crystal molecules. The liquid crystal molecules have a same orientation when not driven by an electric field, this orientation being parallel to the substrates. Pixel electrodes and common electrodes are disposed on the lower substrate. When a voltage is applied to the electrodes, an electric field is generated between the electrodes. The electric field drives the liquid crystal molecules to rotate, so that they have a new orientation that is still parallel to the substrates. The change in orientation results in a change in light transmission. In other words, the operation of the IPS mode LCD is such that the liquid crystal molecules rotate in a plane parallel with the substrates in order to fulfill optical switching. The displayed image has the important advantage of a wide viewing angle. In basic IPS mode LCDs, the pixel electrodes and common electrodes are each comb-shaped. The electric field of these LCDs in a driven state is along a certain direction. When the displayed image is viewed at different oblique angles, an observer can notice a quite large color shift.
Referring to FIG. 6, this is a cross-sectional view of components of a pixel area P of a typical IPS LCD. The pixel area P comprises a gate line 113 arranged in a first direction, a data line 115 and a common line 135 both arranged in a second direction orthogonal to the first direction, a TFT (thin film transistor) 120 positioned at an intersection of the data line 115 and the gate line 113, a pixel electrode 131, and a common electrode 133. The TFT 120 has a gate electrode 121, a source electrode 123 and a drain electrode 125, which are connected with the gate line 113, the data line 115 and the pixel electrode 131 respectively. The pixel electrode 131 and the common electrode 133 are spaced apart from each other. The pixel and common electrodes 131, 133 are each comb-shaped, with the teeth thereof being generally zigzagged. Portions of the teeth of the pixel and common electrodes 131, 133 that are parallel to each other in a third direction form a first sub-electrode group. Portions of the teeth of the pixel and common electrodes 131, 133 that are parallel to each other in a fourth direction form a second sub-electrode group.
When a voltage is applied, because the pixel and common electrodes 131, 133 have zigzagged structures, the electric field (not shown) generated is mainly along two directions. Turning to FIG. 7, the upper portion thereof shows part of the first sub-electrode group, and the lower portion thereof shows part of the second sub-electrode group. The liquid crystal molecules 130 in the upper and lower portions have different orientations, and the LCD exhibits a two-domain display effect. When viewing the LCD display from any oblique angle, the color shifts generated by the two domains counteract, and thus the overall color shift of the display is relatively small.
However, at junctions of the first and second sub-electrode groups, the electric field is abnormal, and the liquid crystal molecules 130 thereat cannot be driven properly. In other words, a disclination of the liquid crystal molecules 130 is generated at the elbows of the teeth of the pixel and common electrodes 131, 133. Light thereat cannot transmit properly, and the contrast ratio of the pixel area is lowered. Furthermore, the two-domain electrode configuration of the LCD inherently limits the display thereof. Equally good visual performance at various different viewing angles cannot be attained.
FIG. 8 shows the results of a simulation of the effect of driving voltage variations on display transmissivity for different rubbing angles formed between a rubbing direction of the liquid crystal molecules 130 and directions defined by the pixel and common electrodes 131, 133. At a larger rubbing angle, the liquid crystal molecules 130 can be driven at a lower threshold driving voltage, and the transmissivity decreases rapidly with an increase in the driving voltage. Therefore, it is necessary to have a precise driving voltage. That is, a precise manufacturing of the TFT 120, the data line 115 and the pixel electrode 131 is necessary. At a smaller rubbing angle, the transmissivity does not decrease with an increasing in the driving voltage. However, a higher threshold driving voltage is needed.
FIG. 9 shows the effect of viewing angle on contrast ratio of the typical IPS LCD described above. The IPS LCD has a largest contrast ratio at the horizontal axis and at the vertical axis. The viewing cone is generally cross-shaped. The contrast ratio is not uniform over different viewing angles.
What is needed, therefore, is an IPS-LCD which has fine viewing characteristics in different viewing directions.